The present invention relates to an electrolyte coated with a roughened interfacial nano-crystalline layer and its use in solid oxide fuel cells.
The use of solid electrolyte materials for fuel cells and oxygen pumps has been the subject of a considerable amount of research for many years. The typical essential components of a solid oxide fuel cell (xe2x80x9cSOFCxe2x80x9d) include a dense, oxygen-ion-conducting electrolyte sandwiched between porous, conducting metal, cermet, or ceramic electrodes. Electrical current is generated in such cells by the oxidation, at the anode, of a fuel material, such as hydrogen, which reacts with oxygen ions conducted through the electrolyte from the cathode.
Practical power generation units will typically include multiple fuel cells of such configuration interconnected in series or parallel with electronically conductive ceramic, cermet, or metal interconnect materials. At the present time, the materials of choice for such devices include yttria-(Y2O3) stabilized zirconia (ZrO2) for the electrolyte, nickel-ZrO2 cermet for the anode material, strontium-doped lanthanum manganite (LaMnO3) for the cathode, and metals, especially Cr/Fe alloys and Ni alloys, intermetallics, and Sr or Ba doped LaCrO3, for interconnect structures. Alternative oxygen ion conductors are known. At sufficient temperatures (e.g., 600xc2x0 C. or above), zirconia electrolytes can exhibit good ionic conductivity but low electronic conductivity.
Several different designs for solid oxide fuel cells have been developed, including, for example, a supported tubular design, a segmented cell-in-series design, a monolithic design, and a flat plate design. All of these designs are documented in the literature, with one recent description in Minh, xe2x80x9cHigh-Temperature Fuel Cells Part 2: The Solid Oxide Cell,xe2x80x9d Chemtech., 21:120-126 (1991).
The tubular design comprises a closed-end porous zirconia tube exteriorly coated with electrode and electrolyte layers. The performance of this design is somewhat limited by the need to diffuse the oxidant through the porous tube. Westinghouse has numerous U.S. patents describing fuel cell elements that have a porous zirconia or lanthanum strontium manganite cathode support tube with a zirconia electrolyte membrane and a lanthanum chromate interconnect traversing the thickness of the zirconia electrolyte. The anode is coated onto the electrolyte to form a working fuel cell tri-layer, containing an electrolyte membrane, on top of an integral porous cathode support or porous cathode, on a porous zirconia support. Segmented designs proposed since the early 1960s (Minh et al., Science and Technology of Ceramic Fuel Cells, Elsevier, p. 255 (1995)), consist of cells arranged in a thin banded structure on a support, or as self-supporting structures as in the bell-and-spigot design.
A number of planar designs have been described which make use of free-standing electrolyte membranes. A cell is formed by applying electrodes and consists of the electrolyte sheet and the applied electrodes. Typically these cells are then stacked and connected in series to build voltage. Monolithic designs, which characteristically have a multi-celled or xe2x80x9choneycombxe2x80x9d type of structure, offer the advantages of high cell density and high oxygen conductivity. The cells are defined by combinations of corrugated sheets and flat sheets incorporating the various electrode, conductive interconnect, and electrolyte layers, with typical cell spacings of 1-2 mm and electrolyte thicknesses of 25-100 microns.
U.S. Pat. No. 5,273,837 to Aitken et al. covers sintered electrolyte compositions in thin sheet form for thermal shock resistant fuel cells. It describes an improved method for making a compliant electrolyte structure wherein a precursor sheet, containing powdered ceramic and binder, is pre-sintered to provide a thin flexible sintered polycrystalline electrolyte sheet. Additional components of the fuel cell circuit are bonded onto that pre-sintered sheet including metal, ceramic or cermet current conductors bonded directly to the sheet as also described in U.S. Pat. No. 5,089,455 to Ketcham et al. U.S. Pat. No. 5,273,837 to Aitken et al. shows a design where the cathodes and anodes of adjacent sheets of electrolyte face each other and where the cells are not connected with a thick interconnect/separator in the hot zone of the fuel cell manifold. These thin flexible sintered electrolyte-containing devices are superior due to the low ohmic loss through the thin electrolyte as well as to their flexibility and robustness in the sintered state.
The performance of a fuel cell, i.e., the current carrying capacity and hence the overall efficiency of the cell, is limited by its internal resistance, the maximum power for any power supply being given by Pmax=V2/4Rinternal. The fuel cell circuit consists of the electrolyte, electrodes, and current conductors. Internal resistance is the sum of several components including the electrode ohmic resistance, the electrolyte resistance, the electrode/electrolyte interfacial resistance to charge transfer reaction, and the current conductor resistance. The interfacial resistance to charge transfer depends mainly on the electrochemical behavior, and physical and chemical nature of the electrode.
Prior art devices have been unable to satisfactorily reduce this interfacial resistance in SOFCs. For example, dense thin layers of CeO2 have been sputtered on zirconia to avoid forming lanthanum zirconate type compounds by reacting with La(Sr)MnO3 during cathode sintering. Lanthanum zirconate compounds have poor ionic conductivity and heavily degrade the performance of the fuel cell when present at the electrode-electrolyte interface. Murray et al. in xe2x80x9cImproved Performance in (La,Sr)MnO3 and (La,Sr)(Co,Fe)O3 Cathodes By the Addition of a Gd-Doped Ceria Second Phasexe2x80x9d, Electrochem. Soc. Proc., PV 99-19: 369-379 (1999) describe the application of ceria-modified cathodes having reduced interfacial resistance to electrolyte surfaces that were grit-roughened to improve electrode film adhesion.
The present invention is directed to providing an improved fuel cell construction, applicable to any of the above fuel cell designs, which provides a cell of improved physical, thermal, and electrical properties. In particular, the present invention is directed to overcoming the performance limitations of high electrode/electrolyte interfacial resistance and poor adhesion between electrodes and electrolytes.
The present invention relates to an electrolyte structure coated on at least one surface with a roughened interfacial nano-crystalline layer.
The present invention also relates to a method of making an electrolyte structure with a roughened interfacial nano-crystalline layer. This method involves providing an electrolyte substrate, applying an interfacial layer of particulates onto at least one surface of the electrolyte substrate, and sintering the interfacial layer.
Another aspect of the present invention is a solid oxide fuel cell which includes a positive air electrode, a negative fuel electrode, an electrolyte structure interposed between the positive air electrode and negative fuel electrode, and a roughened interfacial nano-crystalline layer interposed between the electrolyte structure and at least one of the positive air electrode and negative fuel electrode.
Yet another aspect of the present invention is a solid oxide fuel cell which includes a positive air electrode, a negative fuel electrode, an electrolyte structure interposed between the positive air electrode and negative fuel electrode, wherein the electrolyte structure is bonded to a plurality of electrodes on opposing sides of the electrolyte structure under conditions effective to produce at least two multiple cells connected in series or parallel and wherein the at least two multiple cell fuel cells are combined in an alternating fuel/air manifold wherein similar electrodes of adjacent multiple cell fuel cells face each other under conditions effective to form regions of air or fuel without additional gas separation or interconnection layers, and a roughened interfacial nano-crystalline layer interposed between the electrolyte structure and at least one of the positive air electrode and negative fuel electrode.
The present invention also relates to a method of making a solid oxide fuel cell. This method involves providing an electrolyte substrate, applying a least one interfacial nano-crystalline layer onto at least one surface of the electrolyte substrate, sintering the at least one interfacial nano-crystalline layer, and applying at least one electrode layer to the at least one interfacial nano-crystalline layer.
The roughened interfacial nano-crystalline layer of the present invention reduces interfacial resistance in solid electrolyte fuel cells. Reduced interface resistance leads to higher power densities and/or lower temperature operation of the fuel cell. Thus, the performance of a fuel cell, i.e., the current carrying capacity and hence the overall efficiency of the cell, is improved. In addition, adhesion of electrodes to an electrolyte is improved by use of the roughened interfacial nano-crystalline layer of the present invention.